1. Field of the Invention
The present invention relates to a piston stroke control device which is well suited to control the central position and the amplitude of the stroke of a free piston in a free piston type oscillating compressor.
2. Description of the Prior Art
Stroke control devices for free piston type oscillating compressors have been known from U.S. Pat. Nos. 3,937,600 and 4,067,667. The prior-art devices are so constructed that the diameters of compression pistons are equal on both sides, and the suction pressure and discharge pressure of compression chambers are also equal on both sides. The mean pressure of the gas springs is considered equal on both sides. In practice, however, the mean pressure in compression chambers and gas spring chambers is not exactly the same because of non-uniform piston seals. Accordingly, the central position of the stroke of the free piston deviates somewhat to either side. When the piston seals are conspicuously non-uniform, the free piston deviates extremely so as to render the operation of the device impossible. Regarding the amplitude of the free piston stroke, the amplitude of the alternating component of the piston stroke and the current of a linear motor are detected, and the mean pressure in the gas spring chambers is regulated on the basis of the phase difference between the amplitude of the piston stroke and the current of the linear motor so as to control the piston stroke. Except for a resonant point, the gas springs have two spring constants for the same amplitude. When controlling the piston amplitude, therefore, it must be controlled either in a region greater than or smaller than the resonant point. In the prior art, the phase difference between the piston amplitude and the current of the linear motor at the resonant point is used as reference. Since, however, the phase difference at the resonant point changes depending upon the piston amplitude, the operation of the compressor in the vicinity of the resonant point is impossible when the phase difference is taken as the reference.